An organic light-emitting device has a structure in which two electrodes opposing each other and organic compound layers including a light-emitting layer disposed between these electrodes are layered on a transparent substrate. In the organic light-emitting device, holes and electrons supplied from the electrodes by application of a voltage between the electrodes recombine in the light-emitting layer to generate excitons and give emission of light from the excitons.
The organic light-emitting devices have attracted attention as a technology of the next generation for full color displays having fast response, high luminous efficiency, and flexibility, and material technologies and device technologies for the organic light-emitting devices have been vigorously developed. Among the organic light-emitting devices, in particular, a device utilizing electroluminescence is called an organic electroluminescent (EL) device in some cases.
Recently, in order to enhance the luminous efficiency, organic light-emitting devices employing a system in which phosphorescence through triplet excitons is utilized (hereinafter such organic light-emitting devices are referred to as phosphorescent devices) have been actively developed.
A system in which fluorescence through singlet excitons is utilized is also used, but, in such a system, in principle, only 25% of the excitons generated by recombination of holes and electrons may be used for light emission. On the other hand, in the light emission through triplet excitons, 100% of the excitons may be used for light emission, thus, the luminous efficiency is high.
As light-emitting materials, metal complexes containing iridium (Ir), such as (2-carboxypyridyl)bis(3,5-difluoro-2-(2-pyridyl)phenyl)iridium (FIrpic), are widely used from the viewpoints of material stability and luminous efficiency.
Recently, in addition to luminous efficiency, from the viewpoints of environmental protection and energy saving, a demand for reducing the power consumption of, in particular, displays has been increasing, and development for organic light-emitting devices that are driven by low voltages has been conducted for realizing low-voltage driving of displays.
Furthermore, in the phosphorescent devices, since the device performance is highly influenced by the performance of a host material used together with a light-emitting material (guest material) in the light-emitting layer, the host material has been actively developed.
In order to simultaneously achieving an increase in luminous efficiency and a reduction in voltage of a phosphorescent device, it is necessary that the host material to be used has triplet energy higher than that of the light-emitting material and has high ability of transporting both holes and electrons responsible for excitons to generate light emission. However, as matters now stand, known host materials have not arrived at sufficiently practical levels.
In particular, in blue phosphorescent devices, since the emission peak wavelength of the light-emitting material is short, 450 to 470 nm, the host material is required to have triplet energy higher than that of such a light-emitting material. However, a material that satisfies these strict requirements and has arrived at a practical level has not been found yet.
PTL 1 discloses a compound in which the nitrogen atom of carbazole is substituted with a phenyl group. This carbazole compound has high triplet energy and a high hole-transporting property, but its electron-transporting property is not so high compared to its hole-transporting property. Therefore, the carbazole compound is insufficient for a reduction in voltage.
PTL 2 discloses a device in which a benzimidazole compound is used in the light-emitting layer. However, since the compound disclosed in PTL 2 is not a compound having high triplet energy, when the compound is used in a phosphorescent device, an improvement in luminous efficiency and a reduction in voltage are not achieved.